Frequency “dithering” or “spreading” may be used to reduce peak emitted radiation at any one frequency during electromagnetic interference (EMI) testing and certification of electronic equipment. Dithering may spread the center frequency of clocks used in digital systems, e.g., computers, and/or power supplies for spreading radiated emission energy over a band of frequencies, rather then having all of the emission energy at only one frequency. Thus by spreading the emitted frequency over a band of frequencies, the radio frequency energy at any one frequency may be reduced over a EMI testing time period. Frequency dithering may be accomplished by switching a fundamental frequency, f, between f+f1 and f−f1, where f1 may be a small delta frequency or frequencies.
In dithering the frequency of an oscillator may be shifted from one preset frequency to another preset frequency by changing control bits in a frequency control register. The control bits may control changing of bias currents of the frequency determining circuits of the oscillator. However when abruptly changing these bias currents in the frequency determining circuits of the oscillator, the frequency may shift by more than 300% for a short time duration, e.g., one microsecond, before settling to its final frequency value. Loop response of a phase-locked-loop (PLL) may be fast enough to detect this undesired frequency excursion (frequency glitch) and may propagate it further to a system clock. This may create undesirable effects in the radiated emission (EMI) when used, for example, in switched mode power supply applications.